Direct current conductor corona modelling and metrology

ENGLISH ABSTRACT: Prospects of up-rating existing high voltage direct current (HVDC) transmission schemes, as
well as the conversion of existing alternating current (AC) to direct current (DC) lines and the
development of new HVDC schemes in sub-Saharan Africa, have led to renewed interest in DC
research. The radio interference (RI), audible noise (AN) and corona loss (CL) performance of
HVDC transmission lines are critical factors when assessing the reliability of the line design.
The RI performance is especially important when considering the successful transmission of the
carrier signal of the power line carrier (PLC) system. The PLC system is the main form of
communication between teleprotection devices on the Cahora Bassa HVDC scheme.
The aim of the dissertation is to devise modelling as well as metrological techniques to characterise
DC conductor corona. A particle-in-cell (PIC) computational code is developed to
gain a better understanding of the physical processes that occur during corona events. The
numerical code makes use of the charge simulation method (CSM) and nite element method
(FEM) to solve for the Laplace and Poisson eld equations. Higher-order basis functions are
implemented to obtain a more accurate solution to the Poisson equation. The computational
tool yields insight into the mathematical models for the various ionization, attachment and
electron avalanche processes that give rise to corona currents. Together with a designed and
developed electrometer-type circuit, the numerical code assists the visualisation of the space charge particle dynamics that form in the electrode gap during corona events. The metrological techniques consider the wideband time domain (TD) as well as the frequency
domain (FD) information of the measured corona pulses in the presence of noise. These are
then compared to the narrowband CISPR standard measurements centred around 500kHz. The
importance of impedance matching when attempting to derive a wideband excitation function
is investigated. The TD measurements are quite distinct from the well-published FD measurements,
and consider the pulse shape, pulse spectrum and pulse repetition rates. The use of
three possible conductor corona test methods to study direct current conductor RI performance
under both positive and negative polarities is investigated at high altitude in this dissertation.
These include a small corona cage, a short test line and the Eskom Megawatt Park large outdoor
corona cage. Derived wideband and narrowband monopolar DC RI excitation functions at
500kHz are consolidated with existing radio noise (RN) measurement protocols and prediction
methods.
The use of a corona cage to derive excitation functions for monopolar RI predictions is explored
and it is shown that a small corona cage, due to the build-up of space charge in the
small distance between the electrodes, cannot be used to predict the RI levels on HVDC transmission
lines accurately. As a consequence of the physics, computational modelling and both
frequency and time domain measurements, it is now possible to explain why a small cage system
prevents the accurate RI prediction on transmission lines. The large outdoor corona cage and
short test line RI performance predictions agree with existing empirical prediction formulas.